Fuel cell power plants are well known and are commonly used to produce electrical energy from hydrogen containing reducing fluid and process oxidant reactant streams to power electrical apparatus such as power plants and transportation vehicles. In fuel cell power plants of the prior art, it is well known that, when an electrical circuit connected to the fuel cells is disconnected or opened and there is no longer a load across the cell, such as upon and during shut down of the cell, the presence of air on a cathode electrode along with hydrogen fuel remaining on an anode electrode, often cause unacceptable anode and cathode potentials, resulting in oxidation and corrosion of catalyst and catalyst support materials and attendant cell performance degradation.
Efforts have been proposed to return the cathode electrode to a passive, non-oxidative state upon shut down of the fuel cell. For example, it was thought that inert gas needed to be used to purge both the anode flow field and the cathode flow field immediately upon cell shut down to passivate the anode and cathode electrodes so as to minimize or prevent such cell performance degradation. Further, the use of an inert gas purge avoided, on start-up, the possibility of the presence of a flammable mixture of hydrogen and air, which is a safety issue. While the use of 100% inert gas as the purge gas is most common in the prior art, commonly owned U.S. Pat. Nos. 5,013,617 and 5,045,414 describe using 100% nitrogen as the anode side purge gas, and a cathode side purging mixture comprising a very small percentage of oxygen (e.g. less than 1%) with a balance of nitrogen. Both of these patents also discuss the option of connecting a dummy electrical load across the cell during the start of a purging process to lower the cathode potential rapidly to between the acceptable limit of less than 0.2 volts relative to a hydrogen electrode reference.
A solution has been proposed that avoids the costs associated with storing and delivering a separate supply of inert gas to fuel cells. The costs and complexity of such stored inert gases are undesirable especially in automotive applications where compactness and low cost are critical, and where the system must be shut down and started up frequently. That solution includes shutting down a fuel cell power plant by disconnecting a primary electricity using device (hereinafter, a “primary load”), shutting off the air or process oxidant flow, and controlling the fuel flow into the system and the gas flow out of the system in a manner that results in the fuel cell gases coming to equilibrium across the cells, with the fuel flow shut off, at a gas composition (on a dry basis, e.g. excluding water vapor) of at least 0.0001% hydrogen, balance fuel cell inert gas, and maintaining a gas composition of at least 0.0001% hydrogen (by volume), balance fuel cell inert gas, during shut down. Preferably, any nitrogen within the equilibrium gas composition is from air either introduced into the system directly or mixed with the fuel. This method of fuel cell shut down also includes, after disconnecting the primary load and shutting off the air supply to the cathode flow field, continuing to supply fresh fuel to the anode flow field until the remaining oxidant is completely consumed. This oxidant consumption is preferably aided by having a small auxiliary load applied across the cell, which also quickly drives down the electrode potentials. Once all the oxidant is consumed, the fuel feed is stopped, a fuel exit valve is shut, and air is introduced into the anode flow field (if needed) until the hydrogen concentration in the anode flow field is reduced to a selected intermediate concentration level, above the desired final concentration level. Air flow into the anode flow field is then halted, and the fuel cell gases are allowed to come to equilibrium, which will occur through diffusion of gases across the electrolyte and chemical and electrochemical reaction between the hydrogen and the added oxygen.
During continued shut down, a hydrogen concentration is monitored, and hydrogen is added, as and if necessary, to maintain the desired hydrogen concentration level. That shut down method teaches that a desired range of hydrogen concentration is between 0.0001% and 4%, with the balance being fuel cell inert gases. The latter step of adding hydrogen is likely to be required due to leakage or diffusion of air into the fuel cell and/or leakage or diffusion of hydrogen out of the fuel cell, such as through seals. As air leaks into the system, hydrogen reacts with the oxygen in the air and is consumed. The hydrogen needs to be replaced, from time to time, to maintain the hydrogen concentration within the desired range.
These and other known methods of shutting down a fuel cell power plant require intermittent determinations of gas composition within flow fields of the fuel cells of the plant in particular to determine relative concentrations of oxygen and hydrogen. Known apparatus and methods for determination of such gas compositions within fuel cell flow fields adjacent electrodes involve standard gas composition sensors. Known sensors, however, present significant difficulties in maintaining an efficient shut down of a fuel cell power plant. Known sensors are unreliable, especially within the working environment of a fuel cell flow field adjacent electrodes. For usage within a transportation vehicle that is likely to have between 50,000 to 100,000 shut down and start up cycles during a 10-year expected useful life, sensor reliability is a significant issue. Additionally, securing known gas composition sensors within reactant fluid flow fields or a fuel cell stack within a power plant is a significant manufacturing and cost burden, especially where such fuel cell stack assemblies have over two hundred separate fuel cells. Known gas composition sensors could be secured in reactant fluid flow manifolds of a fuel cell power plant to minimize cost and manufacturing problems. However such placement of sensors external to reactant flow fields would require a recycling flow of gases stored within flow fields in order to measure a representative composition of such reactant gases within the fuel cell flow fields. Such recycling flow would require significant auxiliary power to operate blowers, etc., during shut down of the power plant, which would be an additional burden to the plant.
Consequently, there is a need for a fuel cell power plant that includes an efficient system for monitoring a gas composition within reactant flow fields of fuel cells of the plant during shut down of the plant, and for adjusting the gas composition within the flow fields during shut down to maintain a potential of the electrodes of the fuel cell power plant below an open circuit potential limit.